Comparative evaluation of environment induced cracking of conventional and advanced steam turbine blade steels. Part 1: Stress corrosion cracking

Stress corrosion tests have been conducted to assess the performance of a high strength steel, PH13-8 stainless steel, being considered for advanced steam turbine blades relative to that of a conventional blade steel, FV566. The higher strength steel had a higher threshold stress intensity factor, KISCC, under normal water chemistry conditions (albeit aerated) but in higher chloride solutions the KISCC was relatively low for both alloys with the threshold for the FV566 being slightly lower than that of the PH13-8 steel. It is proposed that hydrogen assisted cracking is the mechanism of failure for both steels. The higher threshold for the PH13-8 steel is considered to be associated with the higher alloying content of the alloy constraining hydrogen uptake as a consequence of enhanced refilming kinetics. The crack growth rate for the higher strength steel was about an order of magnitude greater than that for the FV566 steel, which is consistent with the greater sensitivity to hydrogen once the hydrogen gets into the steel. For both steels, deaeration of the solution to reflect continuous on-load conditions in service caused the growing crack to arrest.